Marvel Ultimate Alliance 2 was, for me, a BIG dissapointment. And as a fan of the first game/Marvel comics in general, I was really looking forward to this one too...
Pros: Graphics (really good character models for most characters), Same fun "party" gameplay as the first, the different choices in conversations (which can be funny), interview with the Hulk over end credits, Hulk and Green Goblin are fun to play as.
Cons: Not as interesting locations to travel to as the first game, Way less characters than the first game, CAN'T USE UNLOCKED CHARACTERS AND COSTUMES UNLESS REPLAYED ON HIGHEST DIFFICULTY LEVEL! (which is brutal), Not as interesting supervillains to beat as the first game, Choice between Pro-and Anti- Reg doesn't feel as important as it should (plus only lasts a handfull of levels), Fusions are boring and underpowered, Only one extra costume for each character, not as much customization for the heroes as the first game.
So, basically, it's exactly like Ultimate Alliance 1 - only much worse. Plus, not being able to keep your unlocked heroes (and costumes) on "heroic" difficulty cripples all replay value. Bottom line - not worth the time. Pick up Ultimate Alliance 1 instead.